IDEAN   23403
INSTITUTO DE ESTUDIOS ANDINOS "DON PABLO GROEBER"
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
A general review about the active tectonics in southern South America
Autor/es:
FOLGUERA, ANDRÉS; GIANNI GUIDO M.; SAGRIPANTI, LUCÍA; ROJAS VERA, EMILIO; BRUNO COLAVITTO; ORTS, DARÍO; RAMOS, VICTOR
Lugar:
San Juan
Reunión:
Congreso; Congreso 27 Asociación Argentina de Geofísicos y Geodestas; 2014
Institución organizadora:
Asociación Argentina de Geofísicos y Geodestas
Resumen:
Active uplift in the Andes has been generally associated with contraction imposed by the convergence between the Pacific subducted plates and the South American plate. However, in the last years multiple mechanisms have been recognized along the Andes that produce, together with orogenic forces, regional to local exhumation of the upper crust. The Andes are formed over a subduction system of two oceanic plates beneath the South American plate. This configuration shows a noticeable symmetry with an Altiplano at its mid sector flanked by two flat subduction settings, the Peruvian in the north and the Pampean-Chilean in the south (Figure 1). The Altiplano region is affected by delamination of the lower lithosphere producing a high topography partially linked to isostatic readjustments. This feature is eastwardly flanked by the Eastern Cordillera and Subandean system that accommodated contraction since the last 10 Ma up to the present. At the Atlantic Brazilian margin another topographic anomaly is recognized in a passive margin (Figure 1). This has been associated with active mountain uplifts restricted to the Atlantic coastal area associated with crustal seismicity and neotectonics. These uplifts are coincident with the area where the Chilean forearc is relatively static respect to the Brazilian Atlantic spreading center. Neotectonic deformation between 27 and 36ºS along the Andes is associated with orogenic mechanisms determined by the Pampean-Chilean flat subduction zone (27-33ºS) and a segment to the south where the Nazca plate changes its angle of subduction smoothly from flat to 30ºE. Here the foreland area is fragmented in a series of active basement blocks such as the Sierras Pampeanas and the San Rafael Block. The area interposed between 36 and 38ºS shows active structures that accommodate shortening and strike-slip displacements at the Payenia volcanic field in the retroarc zone. Evidences of young deformation are found in volcanic products of < 2 Ma age. Part of the retroarc zone is affected by regional uplift linked to the development of extensional troughs such as the Las Loicas extensional system. These systems are spatially linked to a complex pattern of mantle plumes that are impacting the lower crust at the retroarc zone. A main asthenospheric anomaly is branched into a series of minor anomalies that impact the lower crust at the sites of neotectonic activity suggesting a mechanical connection between thermally-weakened crust and horizontal crustal yielding. South 38-39ºS, a slab tear has been described from seismic tomographies after the 27/2/10 Maule earthquake. This slab tear determines a slab window south of 38ºS through a W-NW direction that coincides with the development of the Loncopué trough and an attenuation of the Moho that reduces the Andean roots at these latitudes up to 33 km uplifting and stretching the Agrio fold and thrust belt. Collision of transform zones segmenting the Chile ridge has explained diachronous deformation and exhumation as out-of-sequence thrust fronts from the coastal sectors in the latest Pliocene-early Quaternary, to the arc and retroarc zones up to the Pleistocene an even locally Holocene times. In the last years, geodetic and satellite-gravity (GRACE) measurements associated with the Mw 8.8 Maule earthquake (27/2/2010) have shown the role of large rupture zones along the Pacific subduction zone in the active uplift that affects the area interposed between the coast and the high Andes. On one hand, co-seismic displacements have shown that the coastal areas emerged creating topography. On the other hand, crustal-scale extension affects the upper plate during co- and post-seismic displacements producing regional uplift. The Southern Patagonian region shows particular mechanisms for young to active uplift. Young and buoyant oceanic crust subducts at the Pacific margin decoupling a forearc sliver through the Liquiñe-Ofqui fault zone (LOFZ). This fault zone runs through more than 1,000 km through the arc front accommodating strike-slip to reverse displacements creating topography along the North Patagonian Andes (Figure 1). Its southern edge occurs at the point of collision of the Chile ridge against the trench from which a slab window opens beneath southern Patagonia. The latitudinal extent of this slab window coincides with abnormally high exhumation rates through the Atlantic coast and the scarped morphology of the Patagonian cliffs. Additionally, fluvial fluctuations have been linked to the development of an asthenospheric upwelling coming through the opened window that would be related to surface regional uplift. Finally, localized uplift in the southern extreme of Patagonia is almost entirely linked to the activity of a transform boundary zone between Scotia and South American.